1. Field of the Invention
The present invention relates in general to a method and apparatus for controlling fluid flow using a rotary ball valve. More particularly, the invention relates to a quarter turn rotary ball with cam actuated reciprocating seats for controlling fluid flow in abrasive or high pressure conditions.
2. Description of the Related Art
Certain types of on/off valves have severely attenuated lives due to very concentrated flows with non-axial components during the initial stages of valve opening and final stages of valve closing. In particular, the seats of ball valves and rotary plug valves are susceptible to damage from this type of flow. A second problem which impacts valve actuation is high actuation forces and seat wear due to friction from relative motion of the seats and the sealing plug. This type of problem is common to ball valves, rotary plug valves, and gate valves.
Conventional quarter turn ball valves are commonly used for on/off control of fluid flows. Such conventional valves offer the advantages of simplicity, low fabrication costs, and a full bore flow path with attendant low pressure drops across the flowing valves. The seats of the conventional ball valves are spring loaded and also frequently pressure biased to bear against the ball, but their movement in service is negligible. The conventional valves are directly operated by 90° rotations of their directly connected stems.
However, the conventional valves do not perform well in abrasive flows or under high differential pressures. In such situations, when the valve is rotated sufficiently from its closed position so that an initial flow path is created between the bore of the ball and its seat, a high pressure differential flow is directed across the lenticular gap between the adjacent seating surface and the lip of the through hole of the ball. The same condition occurs during valve closure. The consequence of this situation is rapid erosive wear in the exposed region of the ball and seat. Such wear can quickly cause functional failure of the valve. Abrasive fluids further contribute to such erosive wear.
A critical need exists for a ball valve which is resistant to seat and ball erosion in high pressure and abrasive operation.